The present invention relates to a rubber composition and its manufacturing method and, in more detail, relates to the rubber composition suitable for treads of tires for example, as well as a pneumatic tire obtainable by using such rubber composition. The invention especially relates to those enabling improvement in low-fuel-consumption or braking (wet-road gripping) performance on wet-road, along with wear resistance, while maintaining the braking on wet-road or low-fuel-consumption performance.
Requirement for the low-fuel-consumption performance is ever becoming stricter nowadays; and resultantly, decreasing of rolling resistance of the tire is strictly required. The rolling resistance is related to heat generation of the vulcanized rubber, and thus, in general, it is required to decrease hysteresis loss of the vulcanized rubber or suppressing of loss coefficient. Meanwhile, requirement for safety of the automobile is also high; and thus, required for the rubber composition for the tire tread is not only mere decreasing of the rolling resistance but also maintaining at high levels, of both of the low-fuel consumption and the braking performance.
To cope with such requirements, silica powders as reinforcement fillers are incorporated in the rubber composition with a high content so as to increase the braking on wet-road and low-fuel-consumption performance.
In particular, JP2008-138086A (Japan patent application publication No. 2008-138086) indicates that brittle temperature is kept low by following—“when to obtain a silica-containing rubber composition having natural and/or isoprene rubber and having other diene rubber such as SBR; the other diene rubber is firstly mixed with silica powders and so on to produce a master batch and then, the natural and/or isoprene rubber is added; in a manner to increase non-compatibility between two rubber phases and to thereby increase bipolarity of temperature variance in tan δ” (paragraph 0007).
Meanwhile, JP2006-036918A proposes following—“In first mixing step, a part of to-be-incorporated amount of rubber component is mixed in a tightly closed blender, with silica or other filler, silane coupling agent and other rubber chemicals at preset temperature, within a short time period to obtain a uniform mixture. Then, in second mixing step, obtained mixture is successively (without releasing from the blender) added with remaining part of the rubber component and further mixed for a short time period at a preset temperature” (paragraph 0008). And, in this way, “a master batch of silica-containing rubber composition having excellent dispersion of silica and good performances is obtained highly efficiently without damaging productivity” (paragraph 0008).
JP2008-138081A proposes following. It is aimed that “dispersion of silica powders are improved”, thereby “processability is improved as a result of decreasing viscosity without adding processing-aiding additives” and “chipping resistance is improved” (paragraph 0005). To this ends, “in first mixing step, whole or a part of to-be-incorporated rubber component is mixed with silica and silane coupling agent to obtain a silica master batch. Then, in second mixing step, the master batch is added with remaining part of the rubber component and other ingredients other than vulcanizing chemicals. Subsequently, in last mixing step, the vulcanizing chemicals are added”. Please see paragraph 0008. At the second mixing step, carbon black is added as indicated in claim 1, and all the working examples in tables 1 and 2.
JP1998 (H10)-101849A proposes following. It is aimed that “road-holding performance and running stability are improved without damaging other performances such as wear resistance and blow-out resistance (paragraph 0004).” To this end, “petroleum-derived aromatic hydrocarbon resin having certain extent of kinematic viscosity, as well as carbon black having at least 140 m2/g of specific surface area (N2SA) at nitrogen-gas absorption measurement, are dispersed by wet method into a master batch having styrene-butadiene rubber and carbon black; and such master batch is incorporated into a rubber composition by a certain ratio. Please see paragraph 0005.
Each of above-mentioned JP2006-036918A, JP2008-138081A and JP1998 (H10)-101849A is considered to be intended to efficiently and uniformly disperse silica powders or other reinforcing fillers in rubber. Premise of this is presumably a perception that the silica powder or the like have to be dispersed in rubber as uniformly as possible to achieve maximum contact between the powder and the rubber, in order to achieve a required extent of low-fuel consumption. Moreover, fore-mentioned JP2008-138086A is presumably based on a premise that silica powders or other reinforcing fillers are uniformly dispersed in rubber; and brittle temperature is lowered as a result of non-compatibility among rubber components. Thus, in each of methods of the above-mentioned prior-art documents, it is a premise to uniformly or more uniformly disperse silica or other reinforcing fillers; and each of the methods is thus designed to control compatibility or miscibility among rubber or polymer components.
Meanwhile, any of the above-mentioned methods would not satisfy ever-intensifying requirements for: low-fuel-consumption and braking performance on wet-road while keeping a required level of wear resistance. Above-mentioned JP2008-138081A and JP1998 (H10)-101849A include no mention of low-fuel-consumption performance; and thus, it is assumed that methods of these documents are naturally not adequate for improving the low-fuel-consumption performance.
In view of the above, it is aimed to provide a rubber composition and its manufacturing method as well as a pneumatic tire obtainable by using such rubber composition, which enables improving of the low-fuel-consumption and wear resistances while keeping the braking performance on wet-road, or enables improving of the braking performance on wet-road while keeping the low-fuel-consumption and wear resistance.